1. Field of the Invention
The present invention relates to a photo-crosslinkable resin composition, a hologram recording medium mainly comprising the photo-crosslinkable resin composition, and a hologram recording method using the hologram recording medium. More specifically, it relates to a photo-crosslinkable resin composition and a hologram recording medium having excellent shelf stability and high sensitivity, as well as a hologram recording method by which an excellent volume phase hologram having sufficient durability is provided.
2. Related Background Art
Holography is a technique in which a fringe pattern is constructed in a recording medium by two coherent laser beams and phase or amplitude modulation is added to incident light.
With the advancement of the research regarding the above-mentioned holography technique, various recording media have been suggested, and as materials for use in the recording medium, there have been reported bleached silver salts, dichromated gelatin, thermoplastics, polymeric liquid crystals, inorganic glass materials, ferroelectrics, photopolymers and the like.
Heretofore, as the recording medium by which the high-performance volume phase hologram can be recorded, gelatin sensitized with the dichromate has been well known and used.
This kind of dichromated gelatin recording medium is indeed excellent in that it permits the recording of optical properties with high sensitivity, but the medium has a shortcoming, in that heat resistance, environmental resistance and, particularly, moisture resistance are poor and the formed fringe pattern easily disappears due to the absorption of moisture. Therefore, it is desired to sufficiently enhance the moisture resistance of the hologram. For example, when applied to laser beam-protective spectacles for protecting eyes from laser beams, head-up display devices for cars and the like, the hologram is required to be hermetically sealed with a glass plate or a kind of sealant, and therefore the dichromated gelatin recording medium is unsatisfactory with respect to processability, safety, weight and the like.
In order to overcome the drawbacks of the dichromated gelatin recording medium, photopolymers and the like have been suggested in Japanese Patent Kokai Nos. 2-242817 and 1-502060 and other publications.
These photopolymers are all formed by polymerizing a monomer in a polymer binder, and therefore in the case of a solid phase polymerization, the environmental resistance deteriorates owing to the unreacted monomer. Furthermore, in the reflection hologram, distance of fringe pattern is narrower than in the transmission hologram, and thus, there is a problem that a monomer having a high monomer volume contraction in polymerization is not applicable.
For the purpose of resolving the above-mentioned problems, a technique has been suggested in which a polymer having an aromatic group in a monomer unit, such as poly(vinyl carbazole) (PVCz), is crosslinked by a halogen compound, followed by a development step, to form the hologram (Japanese Patent Kokai No. 2-15269). The PVCz recording medium can provide a volume phase hologram having very excellent moisture resistance, weathering resistance and the like.
However, in the hologram manufacturing process using the PVCz recording medium, there is a problem that a photosensitive solution comprising poly(vinyl carbazole) and carbon tetraiodide has a short pot life, in other words, a problem that the crosslinking reaction of the poly(vinyl carbazole) with carbon tetraiodide tends to proceed not through exposure to a radiation ray, but through exposure to heat. In consequence, the photosensitive solution must be prepared each time coating is to be carried out, and much attention must be paid to the viscosity and sensitivity of the photosensitive solution, as well as to the thickness of the coating film.
In addition, it has been also reported that a halide can be used as a crosslinking agent, and a sensitizing agent such as a cyanine dye or a styryl dye, can be combined with the halide (Japanese Patent Kokai Nos. 2-136858 and 2-136857).
However, these sensitizing agents still have a low sensitivity as regards the desired exposure wave length, and further improvement of the sensitivity is required. Moreover, these sensitizing dyes have a counter anion, and so they show low solubility in an organic solvent, particularly an aromatic solvent which is a good solvent for PVCz. Accordingly, these sensitizing agents are not satisfactory in terms of their solubility in the photosensitive solution.